The stakes are larger than they might appear. As electricity generation expands to meet the demands of inclusive and sustainable societies, high-voltage network infrastructure has accelerated remarkably, and overhead transmission remains far more cost-effective than burying cables underground, a point reinforced by an April 2025 report from the Institution of Engineering and Technology. Insulators in these lines do double duty: they bear mechanical loads while providing sufficient electrical insulation between live conductors and supporting structures. Ceramics and glass once dominated, but polymer composites have steadily displaced them thanks to their low weight, hydrophobicity, breakdown strength, corrosion resistance, low cost, and ease of handling and installation. A composite insulator consists of three sections—a core rod, the housing or weather sheds, and end fittings—and it is the housing that most often determines whether the device survives decades outdoors.

The review highlights data from the International Council on Large Electric Systems (CIGRE) showing that the failure rate of high-voltage composite insulators is driven largely by degradation of the housing and shed. Aging manifests as chalking or powdery surfaces, damage, loss of hydrophobicity, poor mechanical strength, and reduced electrical breakdown strength. Because sheds face environmental, mechanical, electrical, and thermal stress simultaneously, the authors note that shed failure is reportedly inevitable over time, making material improvement the central battleground. Silicone rubber and EPDM have attracted the most attention as matrix materials, and the review systematically examines how each performs and how each can be strengthened.

Silicone rubber is an unusual polymer. Derived ultimately from sand or quartz, it features an alternating silicon-oxygen backbone rather than the carbon-carbon chains of organic rubbers, making it a hybrid organo-silicon material. In high-voltage insulation, the workhorse formulation is based on polydimethylsiloxane, prized for its hydrophobic, water-repellent character. Crucially, SIR can recover hydrophobicity between contamination events and corona exposure, a self-healing surface property that other materials lack. Yet the review stresses that SIR is not invulnerable: silicon-carbon bonds are weaker than the Si-O-Si backbone, opening the door to hydrolysis, oxidative degradation, and photocatalytic degradation under harsh conditions, while ultraviolet radiation can break bonds, lower surface resistance to moisture, and raise leakage currents.

The evidence that fillers can transform SIR is striking. In one study cited by the review, Aiman and colleagues subjected high-temperature vulcanized SIR hybrid composites loaded with nano-silica and micro alumina trihydrate to 900 hours of multi-stress aging. The composite with 6 weight percent SiO2 achieved a direct-current resistivity of about 532.6 × 10^13 ohm-meters at 60 degrees Celsius before aging, with only an 18.5 percent reduction afterward, and maintained the smallest leakage current among all samples. Beyond that optimum, particle agglomeration and poor dispersion degraded performance—illustrating a recurring theme: more filler is not always better. In related work, Wang and colleagues anchored TiO2 nanoparticles onto talc nanoplatelets via a dehydration reaction, producing composites with more uniform particle dispersion and improved breakdown strength, while Mahendra and colleagues found that adding 2 weight percent Al2O3 and 1 weight percent boron nitride to room-temperature vulcanized SIR delayed thermal degradation by roughly 20 degrees Celsius and boosted flashover voltage by 40.5 percent under salt-fog testing of heavily polluted surfaces.

Other SIR studies reinforce the pattern that optimal loading is a narrow target. Rehman and colleagues found that 3 weight percent nano-TiO2 preserved the highest direct-current resistance—from 482.5 down to 198.7 gigaohms even at elevated temperature—while 3 and 5 weight percent loadings showed the best hydrophobicity, attributed to low surface energy, non-polar methyl groups, and surface roughness that hinder water ingress. Ahmed and colleagues observed that dielectric strength in silica-filled HTV-SIR rose with filler content up to 30 weight percent, beyond which conduction paths formed between particles and breakdown strength fell. Hybrid approaches also shine: composites with 4 weight percent nano-SiO2 and 20 weight percent micro-Al2O3 delivered optimal dielectric breakdown strength before and after aging, and a 15 weight percent ATH@SiO2 hybrid in RTV-SIR achieved surface resistivity of 8.4 × 10^14 ohm-centimeters, breakdown strength of 190 kilovolts, and improved tracking resistance.

EPDM tells a parallel story with its own chemistry. Introduced into high-voltage insulator technology some 45 to 50 years ago, this elastomer owes its outstanding environmental resistance to low unsaturation in the parent polymer chain, giving it chemical and weather resistance along with strong thermal and electrical properties. Rizwan and Chandan compared magnesium hydroxide and alumina trihydrate fillers in EPDM and found that Mg(OH)2 composites showed better interfacial interaction, yielding superior tensile strength, thermal stability, and hydrophobicity, while 60 weight percent ATH delivered maximum dielectric breakdown strength. Park and Lee explored a three-component system of surface-modified nano-SiC within micro-SiC/EPDM, where vinyl-functionalized nanoparticles bonded to the matrix through hydrosilylation; adding 2.0 grams of nano-SiC distributed space charge evenly and raised breakdown strength, but beyond that threshold conductive passages formed and performance dropped. Eliyan and colleagues showed that gamma radiation improved ZnO/EPDM dispersion and cross-linking, with tensile strength and dielectric strength peaking at low loadings of 0.1 to 0.3 weight percent.

Hybrid and blended systems are emerging as a particularly promising frontier. Rizwan and Chandan’s accelerated aging work—exposing composites to high temperature, ultraviolet radiation, and acid rain—showed that Mg(OH)2/EPDM composites loaded with TiO2 nanoparticles reached a contact angle of 110 degrees, far above pure EPDM’s 83.6 degrees, because TiO2’s photostability and low surface energy improved matrix interaction, whereas boron nitride, intrinsically partially hydrophilic, left contact angle unchanged. Blending the two rubbers themselves also pays dividends: Alber and colleagues found that a 75 weight percent EPDM, 25 weight percent SIR blend achieved a breakdown strength of 19.61 kilovolts per millimeter, versus 12.89 for the inverse ratio, though tensile strength favored the SIR-rich blend at 3.54 megapascals. Bazli and colleagues similarly reported that EPDM blended into SIR raised dielectric breakdown strength by trapping charge carriers, effectively impeding electrical failure.

The review does not shy away from the field’s persistent problems: filler agglomeration, surface property degradation, corrosion, leakage current, flashover, and discharge all shorten insulator service life, and polymer insulators remain vulnerable to aging under ultraviolet radiation and extreme weather. The authors’ prescriptions are forward-looking. They recommend response surface methodology with central composite design to optimize processing parameters and cost—an approach not yet applied to insulator shed materials—alongside 3D turbula dispersion and sintering techniques to combat nanoparticle agglomeration. They propose fabricating SIR and EPDM composites reinforced with electrical-corrosion-resistant glass nanoparticles, which offer superior dielectric performance, boron-free chemical durability, and high mechanical reinforcement, and they call for wear characterization, long-term dielectric breakdown simulation using COMSOL Multiphysics, and machine learning as a predictive tool. For a component the public rarely sees, the humble rubber shed is suddenly at the center of a materials revolution—and the grid’s reliability may depend on getting every weight percent right.

Subject of Research: Silicone rubber and EPDM composites reinforced with inorganic fillers for high-voltage power transmission insulator sheds

Article Title: Silicone and EPDM rubber-based composites for high-voltage power transmission insulator sheds: a review

Article References: Ogbonna, V., Popoola, O., & Popoola, P. (2026). Silicone and EPDM rubber-based composites for high-voltage power transmission insulator sheds: a review. Polymer Bulletin, 83(12), Article 676. https://doi.org/10.1007/s00289-026-06737-3

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06737-3

Keywords: silicone rubber, EPDM, composite insulators, high-voltage insulation, nanofillers, hydrophobicity, dielectric breakdown, leakage current, flashover, polymer composites, insulator sheds, aging degradation

Flashover is the dividing line between a fire that can be escaped and suppressed and one that consumes the building. In the fire dynamics of enclosed spaces, hot gases accumulate beneath the ceiling, radiate heat downward, and progressively heat all exposed combustible materials. When the upper layer reaches temperatures in the range of roughly 500 to 600 degrees Celsius, or when the heat flux radiated to the floor exceeds approximately 20 kilowatts per square metre, ignition of items throughout the compartment becomes essentially inevitable. In modern buildings, engineers estimate this threshold using decades of compartment-fire experiments. In ancient dwellings, where timber members are massive, ventilation is asymmetric, and ceiling geometries vary from flat smoke lobbies to tall skywell shafts, the standard correlations were never validated, and this is precisely the gap the new work set out to fill.

The study focused on the two most distinctive features of Huizhou vernacular architecture as they are modified during contemporary retrofitting: the partition walls, typically brick or timber panels that subdivide the open halls into rooms and separate storeys, and the skywell, the open vertical courtyard that draws daylight and air into the deep interior of the house. Both features are frequently altered in conservation projects, with additional partitions inserted to create usable rooms and skywells glazed or partially covered to improve thermal comfort. Each change, the researchers found, redistributes airflows and heat in ways that fundamentally reshape when and how flashover occurs.

Using computational fluid dynamics modelling of fire-driven flows, calibrated against the geometry and material properties of representative Huizhou dwellings, the team simulated fire growth in compartments with varying partition configurations and skywell conditions. The simulations tracked the temperature of the hot upper layer, the oxygen concentration, the velocities of air entering and exiting the openings, and the radiative feedback onto the timber surfaces. Across dozens of scenarios, a consistent picture emerged: the timing of flashover is governed by a competition between the rate at which the fire entrains fresh air through the skywell and the rate at which the growing smoke layer is confined by partition walls.

In the original, relatively open configurations, the skywell behaved as a natural chimney. Buoyant gases rose through the vertical shaft and vented to the atmosphere, while cooler air was drawn in at lower levels. This stack-effect ventilation slowed the growth of the upper layer and delayed the onset of flashover, in some scenarios pushing the critical moment well beyond the time available for occupants of a single room to escape. The traditional courtyard house, in this sense, embodies an intuitive, if unintentional, fire-safety design: the same opening that admits light and rain also bleeds away the heat that drives flashover.

The insertion of partition walls changed that calculus dramatically. Because the walls subdivide the interior into smaller, more tightly bounded compartments, the smoke layer within each room reaches ceiling-level confinement much sooner, and the volume of hot gas needed to reach critical temperatures shrinks accordingly. The simulations showed that fires in partitioned rooms could achieve flashover conditions in a markedly shorter time than equivalent fires in open halls, because radiative feedback is concentrated in a smaller enclosure and the combustible timber linings of the room are exposed sooner to intense heating. Yet the walls also play a protective role at the scale of the whole building: once a partitioned room flashed over, the walls acted as thermal barriers, delaying fire spread to adjacent compartments and to the timber structural frame. The trade-off is stark and counterintuitive: retrofitting partitions makes the first room burn faster but may buy the rest of the house critical minutes.

Skywell retrofitting produced the opposite asymmetry. When the courtyard was glazed or covered, the chimney effect was suppressed. Smoke and hot gases that would previously have escaped accumulated instead beneath the new covering, heated it, and spread laterally along the upper storeys of the dwelling. In several scenarios the covering itself, if combustible, became an additional fuel and radiation source. The simulations indicated that covering the skywell could transform a configuration in which flashover remained localized into one in which the entire hall complex approached critical conditions, with the upper layer spilling across partition tops and igniting timber galleries on multiple floors. For conservation authorities weighing the comfort benefits of closing skywells against fire risk, the result offers a quantitative argument for restraint, or at least for ventilated designs that preserve some smoke exhaust capacity.

The researchers distilled these observations into a description of how the critical mechanisms evolve through the fire. In the early growth phase, ventilation dominates: open skywells limit upper-layer development, while partitions do little to impede the initial plume. In the middle phase, confinement dominates: partitions accelerate compartment-level flashover by shrinking the hot-gas volume, while a covered skywell accelerates building-level hazard by trapping effluent. In the late phase, the roles reverse: partitions slow inter-compartment spread and the remaining ventilation paths determine whether the fire becomes ventilation-limited or continues to grow. Flashover, in other words, is not a single threshold but a cascade whose controlling mechanism shifts as geometry and ventilation conditions change, and any retrofit that alters geometry shifts the cascade.

For heritage fire engineering, the practical implications are immediate. The study suggests that retrofit guidelines for vernacular timber dwellings should treat partitions and skywells as a coupled ventilation-fuel system rather than as independent design elements. Inserting partitions without ensuring that each new compartment retains adequate smoke exhaust, for example through high-level vents connected to the skywell shaft, could compress escape times in a way that existing evacuation assumptions do not capture. Conversely, preserving or restoring skywell ventilation may be one of the most effective passive fire protections available in these buildings, requiring no intervention on historic fabric. The work also underscores the value of fire dynamics simulation as a conservation tool, allowing authorities to test candidate retrofits numerically before committing to changes that cannot easily be undone in protected structures.

Beyond Huizhou, the findings speak to a global problem. Vernacular timber architecture across southern China, Japan, the Himalayas and much of Europe shares the same basic ingredients: combustible frames, deep plans, internal courtyards and centuries of accumulated retrofits. Fire remains one of the leading causes of loss of built heritage worldwide, and the pace of climate-driven heatwaves and densifying historic centres is only increasing the exposure. What the Huizhou study demonstrates is that heritage fire safety cannot be imported wholesale from modern codes, because the physics of these buildings is distinctive. It must instead be derived from the buildings themselves, in which the very features that give them their identity may hold the key to their survival, provided that identity is not erased by well-intentioned but fire-illiterate renovation.

Subject of Research: Flashover critical mechanisms under partition-wall and skywell retrofitting in Huizhou ancient dwellings

Article Title: Evolution of flashover critical mechanisms under partition-wall and skywell retrofitting in Huizhou ancient dwellings

Article References: Wu, Y., Li, L., Chen, S., & Zhao, M. (2026). Evolution of flashover critical mechanisms under partition-wall and skywell retrofitting in Huizhou ancient dwellings. npj Heritage Science. https://doi.org/10.1038/s40494-026-03005-5

Image Credits: AI Generated

DOI: 10.1038/s40494-026-03005-5

Keywords: flashover, Huizhou ancient dwellings, fire safety, heritage buildings, partition walls, skywell, timber structures, retrofitting, fire dynamics simulation, cultural heritage, Evolution, critical